Radio apparatus carrying out automatic gain control and gain control method

ABSTRACT

A fading rate of a mobile terminal unit is estimated by a fading rate estimation unit and an optimum step constant is decided by a step constant determiner depending on the estimated fading rate. This step constant is fed back to a feedback data calculator, so that a reception level can be converged to an ideal level even under a fading environment and the like.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radio apparatus and a gain controlmethod, and more particularly to a radio apparatus carrying outAutomatic Gain Control (abbreviated as AGC hereinafter) for a signalfrom a mobile terminal unit in a mobile communication system, and a gaincontrol method for adjusting a gain convergence rate of AGC in the radioapparatus.

2. Description of the Background Art

In a mobile communication system (for example, Personal HandyphoneSystem, abbreviated as PHS hereinafter) which has been rapidly developedin recent years, an approach to extract a signal from a desired mobileterminal unit through adaptive array processing in a radio receivingsystem at a base station is proposed in communication between the basestation and the mobile terminal unit.

The adaptive array processing is processing to remove an interferencecomponent and correctly extract a signal from a desired mobile terminalunit by calculating a weight vector consisting of a receptioncoefficient (weight) for each antenna at a base station based on areceived signal from a mobile terminal unit, for adaptive control.

The adaptive array processing requires a plurality of spatiallydistributed antennas, that is, an array antenna. In an array antenna,for example, formed of two antennas, an array output signal Y(t) isexpressed as follows:Y(t)=W1X1(t)+W2X2(t)

-   -   where X1(t), X2(t) each represents a received signal at each        antenna, and W1, W2 each represents a weight at each antenna.

The received signal at each antenna is expressed as follows:X1(t)=H11S1(t)+H12S2(t)+n1(t)X2(t)=H21S1(t)+H22S2(T)+n2(t)

-   -   where S1(t) represents a signal from a desired mobile terminal        unit and S2(t) represents a signal from an interference-causing        mobile terminal unit.

Here, H11 represents a propagation path property from the desired mobileterminal unit to antenna 1, and H12 represents a propagation pathproperty from the interference-causing mobile terminal unit to antenna1. H21 represents a propagation path property from the desired mobileterminal unit to antenna 2, and H22 represents a propagation pathproperty from the interference-causing mobile terminal unit to antenna2. In addition, n1(t) represents noise at a receiving system of antenna1, and n2(t) represents noise at the system of antenna 2. The arrayoutput in this case is expressed as follows:Y(t)=(W1H11+W2H21)S1(t)+(W1H21+W2H22)S2(t)+W1n1(t)+W2n2(t).

Here, it is assumed that the weight that satisfies the followingequation can be calculated:(W1H11+W2H21)=1(W1H21+W2H22)=0.

Accordingly, the array output signal can be expressed as follows:Y(t)=S1(t)+n(t)

-   -   where n(t)=W1n1(t)+W2n2(t).

Thus, the interference component can be removed and the signal can bereceived from the desired mobile terminal unit by calculating theappropriate weight through the adaptive array processing.

FIG. 11 is a functional block diagram functionally illustrating a radioapparatus that is provided for each antenna at a conventional basestation carrying out the adaptive array processing using such aplurality of antennas and carries out AGC for a signal from a mobileterminal unit. FIG. 12 is a flow chart illustrating a gain controlmethod for adjusting a gain convergence rate of AGC in such a radioapparatus.

First referring to FIG. 11, a signal received from an antenna 1 isamplified by an AGC amplifier 2, converted into an IQ signal formed ofan In-phase component (I component) and Quadrature component (Qcomponent) by a quadrature detector 3, and thereafter stored into memory4.

IQ signal once stored in memory 4 is provided to a demodulation circuit5. Demodulation circuit 5 also receives IQ signal from another antenna(not shown), performs the adaptive array processing as described aboveand demodulation processing and extracts the signal from each mobileterminal unit.

A reception level detection unit 6 obtains the reception level of thesignal from IQ signal stored in memory 4. An amplitude value of IQsignal is calculated, for example, for eight symbols from 60th symbol ofthe received signal for each frame. The maximum amplitude value forthese eight symbols is considered as the reception level for that frame.

A feedback data calculator 7 calculates feedback data that decides anamplitude ratio of AGC amplifier 2 in the next frame by the receptionlevel obtained by reception level detection unit 6 and a step constantstored in memory 9. Here, when the reception level obtained by receptionlevel detection unit 6 is represented by P_max, a prescribed ideal valueis represented by P_ideal, and the step constant is represented by Step,the amount of change ΔFB from feedback data at the time of receiving theprevious frame to feedback data at the time of receiving the next framecan be calculated by the following equation:ΔFB=(P_max−P_ideal)/2^(Step).

When the value of the feedback data at the time of receiving theprevious frame is represented by FB, feedback data FB′ at the time ofreceiving the next frame can be expressed as follows:FB′=FB−ΔFB.

The feedback data calculated by feedback data calculator 7 is oncestored in memory 8. The stored feedback data is read in the next frameand provided to a gain control input of AGC amplifier 2 to be reflectedin AGC at the time of receiving the next frame.

Referring to FIG. 12, the gain control method for adjusting the gainconvergence rate of AGC in the radio apparatus shown in FIG. 11 will nowbe described. It is noted that the following process is implemented by aDigital Signal Processor (DSP) of the radio apparatus in a softwaremanner.

At step S1, the signal from the mobile terminal unit is subjected toquadrature detection. Here, an RXIF signal that is an intermediatefrequency signal received from the mobile terminal unit is converted toan RXIQ signal subjected to quadrature detection.

At step S2, such a symbol is set in that the reception level of thesignal received from the mobile terminal unit starts to be detected. Anexample is herein shown where the reception level is detected from 60thsymbol of the received signal.

At step S3, it is determined whether the present symbol is in a symbolperiod in which the reception level of the signal from the mobileterminal unit is detected. For example, when the reception level isdetected in the 8-symbol period from the 60th symbol to the 67th symbol,if the symbol in which amplitude will be calculated from now on precedesthe 68th symbol, a process of calculating the amplitude in that symbolwill follow, and if not, a process of calculating the feedback data willfollow.

At step S4, the amplitude in that symbol is calculated. The value ofsquared I component of IQ signal is added to the value of squared Qcomponent of IQ signal. Here, in order to simplify the process, thesquare roots of the resulting sum is not obtained.

At step S5, it is determined whether or not amplitude A calculated atstep S4 is greater than the maximum amplitude A_max stored until now.

At step S6, if it is determined that A is greater than A_max at step S5,A_max is replaced by A.

At step S7, the symbol in which the amplitude is calculated is shiftedby one in ascending order.

At step S8, the amount of change of the feedback data is calculated fromthe maximum amplitude value of the amplitudes from the 60th symbol to67th symbol and from a fixed step constant stored in the memory.

At step S9, the feedback data in the next frame is calculated and thecalculated feedback data is reflected in AGC at the time of receivingthe next frame.

In the adaptive array processing, weights are calculated such that thepower of interference component is small when the signals received froma plurality of antennas are multiplied by respective weights to besynthesized. Specifically, the operation of multiplying the weight meansthat the amplitude and phase of the signal received from each antennaare adjusted appropriately. Therefore, if the amplitude and phaseinformation of the received signal suffers an error due to waveformdistortion and the like, the adaptive array processing cannot fullyfunction.

On the other hand, as to the input/output characteristics of generalamplifiers, output at a certain level causes saturation, therebycreating a non-linear region. Therefore, in a case where the feedbackdata is calculated based on the fixed step constant as described above,for example, if the reception level of the signal from the mobileterminal unit is largely fluctuated by the effects of a distance betweenthe base station and the mobile terminal unit, any possible obstacle,fading, and the like, the amplifier (AGC amplifier 2) at the basestation enters into this non-linear region, leading to distortion of thereceived waveform, thereby resulting in that the adaptive arrayprocessing cannot fully function.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a radioapparatus and a gain control method allowing for an adaptive arrayprocessing without a non-linear operation of an amplifier even under anenvironment in which a reception level of a signal from a mobileterminal unit largely fluctuates due to the effects of a distancebetween a base station and the mobile terminal unit, any obstacle,fading, and the like.

In accordance with the present invention, a radio apparatus carrying outAGC for a signal from a mobile terminal unit includes: a gain controlfeedback circuit performing AGC on the signal depending on a level ofthe signal; a fading rate estimation unit estimating a fading rate ofthe mobile terminal unit; and a step constant determiner determining anoptimum step constant which decides a gain of the gain control feedbackcircuit depending on the fading rate estimated by the fading rateestimation unit. The gain control feedback circuit includes a feedbackdata calculator calculating feedback data which defines a gain of thegain control feedback circuit based on the signal level and the optimumstep constant as determined by the step constant determiner.

Preferably, the fading rate estimation unit calculates a receptionresponse vector of the signal from the mobile terminal unit andestimates a fading rate from a correlation value of the receptionresponse vectors between frames.

More preferably, the step constant determiner decides an optimum stepconstant at the fading rate estimated by the fading rate estimation unitbased on an association table of preliminarily obtained fading rates andstep constants.

More preferably, the feedback data calculator calculates feedback datain a next frame by obtaining a value of difference between a value ofthe signal level and an ideal value, dividing the value of difference bya prescribed constant a number of times corresponding to the stepconstant, followed by multiplication by a prescribed coefficient,thereby calculating an amount of change in feedback data, and adding theamount of change to feedback data at the time of receiving a previousframe.

In accordance with another aspect of the present invention, a radioapparatus carrying out AGC for a signal from a mobile terminal unitincludes: a gain control feedback circuit performing AGC on the signaldepending on a level of the signal; a received signal strength detectionunit detecting a received signal strength of the signal from the mobileterminal unit, at an antenna end of the radio apparatus; a feedback dataconversion unit carrying out conversion from the received signalstrength detected by the received signal strength detection unit tooptimum feedback data which decides a gain of the gain control feedbackcircuit based on an association table of preliminarily obtained receivedsignal strengths and feedback data; and a feedback data switch unitselecting the optimum feedback data converted by the feedback dataconversion unit as an initial value of feedback data of the gain controlfeedback circuit.

In accordance with a further aspect of the present invention, a radioapparatus carrying out AGC for a signal from a mobile terminal unitincludes a gain control feedback circuit performing AGC on the signaldepending on a level of the signal. The gain control feedback circuitincludes a first gain control circuit supplying first feedback dataallowing the level of the signal to converge to an ideal value slowly,and a second gain control circuit supplying second feedback dataallowing the level of the signal to converge to an ideal value fast. Theradio apparatus further includes a received signal strength detectionunit detecting a received signal strength of the signal from the mobileterminal unit, at an antenna end of the radio apparatus, and a feedbackdata switch unit selecting the first feedback data or the secondfeedback data depending on a detection result from the received signalstrength detection unit.

Preferably, the feedback data switch unit makes a switch from the firstfeedback data to the second feedback data when fluctuation of thereceived signal strength detected by the received signal strengthdetection unit decreases to a prescribed threshold or below or increasesto a prescribed threshold or above.

In accordance with yet another aspect of the present invention, a radioapparatus carrying out AGC for a signal from a mobile terminal unitincludes a gain control feedback circuit performing AGC on the signaldepending on a level of the signal. The gain control feedback circuitincludes a first gain control circuit supplying first feedback dataallowing the level of the signal to converge to an ideal value slowly,and a second gain control circuit supplying second feedback dataallowing the level of the signal to converge to an ideal value fast. Theradio apparatus further includes a new terminal detector detectingwhether a signal is received from a new mobile terminal unit, and afeedback data switch unit selecting the first feedback data or thesecond feedback data depending on a detection result from the newterminal detector.

Preferably, the new terminal detector includes a demodulation circuitdemodulating a specific signal from a new mobile terminal unit and anerror determiner determining whether an error exists in the signaldemodulated by the demodulation circuit and if no error exists,determining that a signal is received from a new mobile terminal unit.

More preferably, the error determiner determines that a signal has beenreceived from a new mobile terminal unit if a unique word error does notexist in the signal demodulated in the demodulation circuit.

More preferably, the error determiner determines that a signal has beenreceived from a new mobile terminal unit if the number of bit errors ina preamble portion is equal to or smaller than a prescribed threshold inthe signal demodulated in the demodulation circuit.

More preferably, the gain control feedback circuit includes a receptionlevel detection unit detecting the level of the signal. The receptionlevel detection unit calculates an amplitude value for each IQ signalduring a prescribed period of a received signal and regards a maximumamplitude value among the amplitude values as a reception level of thereceived signal.

More preferably, the gain control feedback circuit includes a receptionlevel detection unit detecting the level of the signal. The receptionlevel detection unit calculates an amplitude value for each IF signalduring a prescribed period of a received signal and regards a maximumamplitude value among the amplitude values as a reception level of thereceived signal.

In accordance with another aspect of the present invention, a gaincontrol method in a radio apparatus carrying out AGC for a signal from amobile terminal unit includes the steps of: performing AGC on the signaldepending on a level of the signal; estimating a fading rate of themobile terminal unit; and determining an optimum step constant whichdecides a gain of AGC, depending on the estimated fading rate. The stepof carrying out AGC includes the step of calculating feedback data whichdefines a gain of AGC, based on the signal level and the optimum stepconstant as determined.

Preferably, the step of estimating a fading rate includes the step ofcalculating a reception response vector of the signal from the mobileterminal unit and estimating a fading rate from a correlation value ofthe reception response vectors between frames.

More preferably, the step of determining an optimum step constantincludes the step of deciding an optimum step constant at the estimatedfading rate based on an association table of preliminary obtained fadingrates and step constants.

More preferably, the step of calculating feedback data includes the stepof calculating feedback data in a next frame by obtaining a value ofdifference between a value of the signal level and an ideal value,dividing the value of difference by a prescribed constant a number oftimes corresponding to the step constant, followed by multiplication bya prescribed coefficient, thereby calculating an amount of change offeedback data, and adding the amount of change to feedback data at thetime of receiving a previous frame.

In accordance with a further aspect of the present invention, a gaincontrol method in a radio apparatus carrying out AGC for a signal from amobile terminal unit includes the steps of: performing AGC on the signaldepending on a level of the signal; detecting a received signal strengthof the signal from the mobile terminal unit at an antenna end of theradio apparatus; carrying out conversion from the detected, receivedsignal strength to optimum feedback data which decides a gain of AGCbased on an association table of preliminarily obtained received signalstrengths and feedback data; and selecting the converted optimumfeedback data as an initial value of the feedback data.

In accordance with yet another aspect of the present invention, a gaincontrol method in a radio apparatus carrying out AGC for a signal from amobile terminal unit includes the step of performing AGC on the signaldepending on a level of the signal. The step of performing AGC includesa first step of supplying first feedback data allowing the level of thesignal to converge to a ideal value slowly, and a second step ofsupplying second feedback data allowing the level of the signal toconverge to an ideal value fast. The method further includes the stepsof detecting a received signal strength of the signal from the mobileterminal unit, at an antenna end of the radio apparatus, and selectingthe first feedback data or the second feedback data depending on adetection result.

Preferably, the step of selecting feedback data includes the step ofmaking a switch from the first feedback data to the second feedback datawhen fluctuation of the detected, received signal strength decreases toa prescribed threshold or below or increases to a prescribed thresholdor above.

In accordance with a still further aspect of the present invention, again control method in a radio apparatus carrying out AGC for a signalfrom a mobile terminal unit includes the step of performing AGC on thesignal depending on a level of the signal. The step of performing AGCincludes a first step of supplying first feedback data allowing thelevel of the signal to converge to an ideal value slowly, and a secondstep of supplying second feedback data allowing the level of the signalto converge to an ideal value fast. The method further includes thesteps of detecting whether a signal is received from a new mobileterminal unit, and selecting the first feedback data or the secondfeedback data depending on a detection result.

Preferably, the step of detecting a new mobile terminal unit includesthe steps of demodulating a specific signal from a new mobile terminalunit, and determining whether an error exists in the demodulated signaland if no error exists, determining that a signal is received from a newmobile terminal unit.

More preferably, the step of determining whether an error existsincludes the step of determining that a signal has been received from anew mobile terminal unit when a unique word error does not exist in thedemodulated signal.

More preferably, the step of determining whether an error existsincludes the step of determining that a signal has been received from anew mobile terminal unit if the number of bit errors in a preambleportion is equal to or smaller than a prescribed threshold in thedemodulated signal.

More preferably, the step of performing AGC includes the step ofdetecting a level of the signal. The step of detecting a level of thesignal includes the step of calculating an amplitude value for each IQsignal during a prescribed period of a received signal and regarding amaximum amplitude value among the amplitude values as a reception levelof the received signal.

More preferably, the step of performing AGC includes the step ofdetecting a level of the signal. The step of detecting a level of thesignal includes the step of calculating an amplitude value for each IFsignal during a prescribed period of a received signal and regarding amaximum amplitude value among the amplitude values as a reception levelof the received signal.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram functionally illustrating a radioapparatus in accordance with a first embodiment of the presentinvention.

FIG. 2 is a flow chart illustrating a gain control method in accordancewith the first embodiment of the present invention.

FIG. 3 is a functional block diagram functionally illustrating the radioapparatus in accordance with a second embodiment of the presentinvention.

FIG. 4 is a flow chart illustrating the gain control method inaccordance with the second embodiment of the present invention.

FIG. 5 is a functional block diagram functionally illustrating the radioapparatus in accordance with a third embodiment of the presentinvention.

FIG. 6 is a flow chart partially illustrating the gain control method inaccordance with the third embodiment of the present invention.

FIG. 7 is a flow chart partially illustrating the gain control method inaccordance with the third and fourth embodiments of the presentinvention.

FIG. 8 is a functional block diagram functionally illustrating the radioapparatus in accordance with the fourth embodiment of the presentinvention.

FIG. 9 is a flow chart illustrating the gain control method inaccordance with the fourth embodiment of the present invention.

FIG. 10 is a functional block diagram functionally illustrating theradio apparatus in accordance with a fifth embodiment of the presentinvention.

FIG. 11 is a functional block diagram functionally illustrating aconventional radio apparatus carrying out AGC for a signal from a mobileterminal unit.

FIG. 12 is a flow chart illustrating a gain control method for adaptinga gain convergence rate of AGC in the conventional radio apparatus shownin FIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention will be describedin detail with reference to the figures. It is noted that in the figuresthe same or corresponding parts will be denoted with the same referencecharacters and description thereof will not be repeated.

(First Embodiment)

FIG. 1 is a functional block diagram functionally illustrating a radioapparatus in accordance with a first embodiment of the presentinvention. FIG. 2 is a flow chart illustrating a gain control method inaccordance with the first embodiment of the present invention.

The radio apparatus shown in FIG. 1 is same as the radio apparatus shownin FIG. 11 except the following point. Specifically, the radio apparatusshown in FIG. 1 includes a fading rate estimation unit 10 and a stepconstant determiner 11 in place of memory 9 in FIG. 11.

Fading rate estimation unit 10 estimates a fading rate by a correlationvalue of reception response vectors of the signal from a mobile terminalunit.

The reception response vector is calculated by fading rate estimationunit 10 as follows. For the purpose of illustration, it is assumed thata signal from a desired mobile terminal unit is received at an arrayantenna formed of two antennas. The signal from the mobile terminal unitis represented by S1(t), the signal received from an antenna 1 isrepresented by X1(t), the signal received from another antenna (notshown) is represented by X2(t), a noise component of antenna 1 isrepresented by n1(t), and a noise component from another antenna isrepresented by n2(t).

Here the received signal at each antenna is expressed as follows:X1(t)=h1S1(t)+n1(t)X2(t)=h2S1(t)+n2(t).

Here, h1 and h2 are parameters expressed in complex numbers includingamplitudes and phases, representing a propagation path from the desiredmobile terminal unit to antenna 1 and a propagation path from thedesired mobile terminal unit to another antenna, respectively.

Here, reception response vector H or signal S1(t) is expressed in thefollowing equation:H=[h1, h2]T(T is transpose).

In this case, there is no correlation between S1(t), n1(t) and n2(t).

Furthermore, assuming that signal S1(t) is created as a reference signalr1(t), h1(t) is calculated based on the following equation, bymultiplying received signal X1(t) by reference signal r1*(t) (* is acomplex conjugate) and obtaining an ensemble-average: $\begin{matrix}{{E\left\lbrack {{{X1}(t)}{{r1}(t)}} \right\rbrack} = {E\left\lbrack {{{X1}(t)}{{S1}(t)}} \right\rbrack}} \\{= {{E\left\lbrack {{{h1S1}(t)}{{S1}^{+}(t)}} \right\rbrack} + {E\left\lbrack {{{n1}(t)}{{S1}^{+}(t)}} \right\rbrack}}} \\{= {{{h1E}\left\lbrack {{{S1}(t)}{{S1}^{+}(t)}} \right\rbrack} + {E\left\lbrack {{{n1}(t)}{{S1}^{+}(t)}} \right\rbrack}}} \\{= {{h1}.}}\end{matrix}$

Here, the ensemble-average of the identical signals is one and theensemble-average of the signals not correlated is approximately zero, sothat E[S1(t)S1*(t)]=1, E[n1(t)S1*(t)]≈0.

Similarly, h2 is calculated by multiplying received signal X2(t) byreference signal r1*(t) and obtaining an ensemble-average. Thusreception response vector H of signal S1(t) from the desired mobileterminal unit is calculated.

In this manner, fading rate estimation unit 10 first calculates thereception response vector of the signal from the mobile terminal unitand further calculates a fading rate based on the calculated receptionresponse vector. A method of estimating a fading rate will be describedbelow.

The reception response vector of the signal from the mobile terminalunit in one frame is represented by H(f), and the reception responsevector from the mobile terminal unit in the next frame is represented byH(f+1). As described above, the reception response vector indicates thepropagation path from the mobile terminal unit to the base station. Whenthe fading rate is low, H(f) and H(f+1) have a similar value and thecorrelation value of the reception response vectors becomes greaterbecause fluctuations in the propagation path are small. On the contrary,when the fading rate is high, H(f) and H(f+1) have different values andthe correlation value of the reception response vectors becomes smallerbecause fluctuations in the propagation path are large.

Then, the correlation value between frames of the reception responsevectors of the signal from the mobile terminal unit is obtainedbeforehand for each of a plurality of fading rates, and a tableassociating the fading rates with the correlation values of thereception response vectors is stored in memory (not shown), which isprovided in fading rate estimation unit 10. In actual communication,fading rate estimation unit 10 calculates the reception response vectorof the signal from the mobile terminal unit every frame through themethod described above and obtains the correlation value of thereception response vectors between frames. The fading rate of the mobileterminal unit can be estimated from that correlation value withreference to the association table with the fading rates as describedabove.

Step constant determiner 11 decides an optimum step constant dependingon the fading rate of the mobile terminal unit that is estimated byfading rate estimation unit 10.

The optimum step constant depending on the fading rate is decided asfollows. First, the reception levels of the signal from the mobileterminal unit at a fading rate are observed for a prescribed number offrames with the step constants being varied, and a step constant havinga reception level closest to an ideal value with smallest dispersion isregarded as the optimum step constant at that fading rate.

The table in which the fading rates are associated with the optimum stepconstants can be created by performing the following procedures on aplurality of fading rates. This association table is stored in memory orthe like (not shown) to form step constant determiner 11. The stepconstant read from step constant determiner 11 is applied to feedbackdata calculator 7.

Referring to FIG. 2, the gain control method in accordance with thefirst embodiment will now be described. The flow chart in FIG. 2 isdifferent from the flow chart of the conventional example shown in FIG.12 in the following points.

First at step S10, a reception response vector is calculated based on asignal from a mobile terminal unit.

At step S11, a fading rate is calculated based on the reception responsevector calculated at step S10.

At step S12, a step constant is decided based on the fading ratecalculated at step S11.

In this manner, in accordance with the first embodiment, non-linearoperation of AGC amplifier 2 due to the effect of fading can beprevented by deciding a step constant depending on the fading raterather than a fixed step constant.

(Second Embodiment)

FIG. 3 is a functional block diagram functionally illustrating the radioapparatus in accordance with a second embodiment of the presentinvention. FIG. 4 is a flow chart illustrating the gain control methodin accordance with the second embodiment of the present invention.

First referring to FIG. 3, the signal received from antenna 1 isamplified by AGC amplifier 2, converted to IQ signal by quadraturedetector 3 and thereafter stored in memory 4.

An RSSI detection unit 16 detecting a received signal strength (forexample, a received signal strength indicator: abbreviated as RSSIhereinafter) detects the RSSI of the signal received from the mobileterminal unit at antenna 1 in real time.

A feedback data conversion unit 17 obtains associated feedback data fromthe RSSI of the signal from the mobile terminal unit that is detected byRSSI detection unit 16. A feedback data switch unit 18 operates toselect and provide the output of feedback data conversion unit 17 to thegain control input of AGC amplifier 2 at the start of reception. Aninitial value of AGC gain is thus set.

Feedback data conversion unit 17 is configured as follows. The feedbackdata is varied with the signal level from the mobile terminal unit beingkept constant, and feedback data is obtained such that the receptionlevel of the signal that has passed through AGC amplifier 2 attains anideal value. The associated feedback data is obtained with varying thelevel of the signal from the mobile terminal unit which is keptconstant. An association table of RSSI and feedback can thus be createdand stored in memory or the like (not shown). In this manner, feedbackdata conversion unit 17 can be configured.

Reception level detection unit 12 calculates a reception level with IQsignal that has passed through quadrature detector 3. The amplitudes ofIQ signal are calculated, for example, during a 4-sample period aftersetting of the initial value, and the maximum amplitude value amongthese is regarded as the reception level at that moment.

A feedback data calculator 13 calculates feedback data with thereception level obtained by reception level detection unit 12 and thestep constant stored in memory 15. Here, the amount of change ΔFB fromfeedback data at the time of receiving a previous frame to feedback dataat the time of receiving a next frame can be calculated as follows:ΔFB=(P_max−P_ideal)/2^(Step)

-   -   where the reception level obtained by reception level detection        unit 12 is represented by P_max, a prescribed ideal value by        P_ideal, and the step constant by Step.

Therefore, when the value of the feedback data at the time of receivingthe previous frame is represented by FB, the feedback data FB′ at thetime of receiving the next frame can be calculated as follows:FB′=FB−ΔFB.

The feedback data calculated by feedback data calculator 13 is oncestored in memory and thereafter provided to the gain control input ofAGC gain amplifier 2 through feedback data switch unit 18. Since theinitial value of AGC gain of AGC amplifier 2 is properly set, thefeedback data is immediately reflected in AGC.

Referring to FIG. 4, the gain control method in accordance with thesecond embodiment will now be described.

First at step S13, RSSI of a signal received from a mobile terminal unitat an antenna end is detected.

At step S14, conversion from the RSSI detected at step S13 to feedbackdata is carried out. With reference to the memory storing theassociation table of RSSI and feedback data, feedback data associatedwith the RSSI detected at step S13 is retrieved.

At step S15, the feedback data obtained at step S14 is immediatelyreflected in AGC. More specifically, the feedback data obtained at stepS14 is provided to the gain control input of AGC amplifier 2 as aninitial value.

At step S16, s indicative of the nth sample at which calculation ofamplitude value starts is set. More specifically, at step S16, theordinal position of the sample at which a burst rising of the signalfrom the mobile terminal unit is detected is set. Here, it is assumedthat the burst rising is detected at the 40th sample.

At step S17, j represents the number of times feedback data is set bydetecting the reception level from the mobile terminal unit.

At step S18, if the number of times feedback data is set by detectingthe reception level is not less than three, the process will end, and ifnot, the subsequent process will follow.

At step S19, it is determined whether the present sample is in a sampleperiod during which the reception level of the signal from the mobileterminal unit is detected. In a case where the reception level isdetected during an 8-sample period from the sample specified by s, forexample, if the sample at which an amplitude will be calculated precedesthe eighth sample from the sample specified by s, a process ofcalculating the amplitude of that sample will follow. If not, a processof calculating the feedback data will follow.

At step S20, the amplitude of the corresponding symbol is calculated.The value of squared I component of IQ signal is added to the value ofsquared Q component of IQ signal. Here, for the purpose of simplifyingthe process, the square roots of the resulting sum is not obtained.

At step S21, it is determined whether amplitude A calculated at step S20is greater than the maximum amplitude A_max stored until now.

At step S22, if it is determined that A is greater than A_max at stepS21, A_max is replaced by A.

At step S23, the sample at which the amplitude is calculated is shiftedby one in ascending order.

At step S24, the amount of change of the feedback data is calculatedbased on the maximum amplitude value during the 8-sample period from thesample specified by s and the step constant.

At step S25, the feedback data in the next frame is calculated and thecalculated feedback data is immediately reflected in AGC.

At step S26, the sample at which amplitude is calculated next is set.

At step S27, the number of times the feedback data is set is updated.

In this manner, according to the second embodiment, the initial value ofAGC gain is set depending on RSSI at the start of reception, so that thefeedback data calculated by feedback calculator 13 and feedback dataconversion unit 17 can be immediately reflected in AGC.

(Third Embodiment)

FIG. 5 is a functional block diagram functionally illustrating the radioapparatus according to a third embodiment of the present invention. FIG.6 is a flow chart illustrating a part of the gain control method inaccordance with the third embodiment of the present invention. FIG. 7 isa flow chart illustrating the remaining part of the gain control methodin accordance with the third embodiment of the present invention.

First referring to FIG. 5, RSSI detection unit 16 detects the RSSI ofthe signal received at antenna 1 as illustrated with reference to FIG.3.

A feedback data switch unit 19 provides feedback data read from thefeedback data conversion unit 17 at the start of reception to the gaincontrol input of AGC amplifier 2 as an initial value depending on RSSIof the signal from the mobile terminal unit and switches the feedbackdata input into the gain control input of AGC amplifier 2 from a firstgain control feedback unit in which the reception level converges slowto a second gain control feedback unit in which reception levelconverges fast. The switching is carried out, for example, when the RSSIdecreases by 20 dBμV or more or increases by 3 dBμV or more as comparedwith the previous frame.

It is noted that the first gain control feedback unit is basicallyformed of AGC amplifier 2, reception level detection unit 6, feedbackdata calculator 7, memory 8 and memory 9, and adapts the calculatedfeedback data to a reception value for the next frame. Furthermore, thesecond gain control feedback unit is basically formed of AGC amplifier2, reception level detection unit 12, feedback data calculator 13,memory 14 and memory 15, and immediately adapts the calculated feedbackdata to the frame being received at present. The difference in theconvergence rate results from the difference in the step constantsstored in memories 9, 15.

Next referring to FIGS. 6 and 7, the gain control method in accordancewith the third embodiment will be described.

At step S28 in FIG. 6, the RSSI of the signal received at the antennaend from the mobile terminal unit that is detected at step S13 iscompared between frames. In this case, the present RSSI is compared tothe stored RSSI of the signal received one frame before that frame. Ifthe amount of increase is greater than a prescribed threshold A, forexample, greater than 5 dBμV, or if the amount of decrease is greaterthan a prescribed threshold B, for example, greater than 10 dBμV, thegain control in which the reception level converges to an ideal valuefast is carried out (see FIG. 7). If not, the gain control in which thereception level converges to an ideal value slowly is carried out (seeFIG. 6). It is noted that the process in FIG. 6 is approximately same asthe conventional example in FIG. 12, and the process in FIG. 7 isapproximately same as the second embodiment in FIG. 4. Therefore,description of these processes is not repeated.

In this manner, in accordance with the third embodiment, the gainconvergence rate of AGC can be optimized by switching the feedback datato AGC amplifier 2 depending on RSSI.

(Fourth Embodiment)

FIG. 8 is a functional block diagram functionally illustrating the radioapparatus in accordance with a fourth embodiment of the presentinvention. FIG. 9 is a flow chart partially illustrating the gaincontrol method in accordance with the fourth embodiment of the presentinvention. It is noted that the radio apparatus shown in FIG. 8 is sameas the radio apparatus in accordance with the third embodiment shown inFIG. 5 except the following points.

First referring to FIG. 8, a feedback data switch unit 20 makes a switchfrom the first gain control feedback unit in which convergence of thereception level is slow to the second gain control feedback unit inwhich convergence of the reception level is fast, when it is determinedthat the signal from a new mobile terminal unit is received, in additionto setting of an initial value of feedback data based on RSSI describedin the second embodiment.

It is determined that a signal is received from a new mobile terminalunit as follows. When the signal is regarded as being received from anew mobile terminal unit and is demodulated, resulting in that there isno unique word error and the number of bit errors in a preamble portionis less than a prescribed number of bits, feedback data switch unit 20determines that a signal is being received from the new mobile terminalunit and switches the gain control method.

Referring now to FIGS. 7 and 9, the gain control method in accordancewith the fourth embodiment will be described.

At step S29 in FIG. 9, upon regarding a signal as being received from anew mobile terminal unit, demodulation is carried out.

At step S30, if the signal demodulated at step S29 has no unique worderror or if the number of bit errors in a preamble portion is not morethan a prescribed threshold, for example, two bits or less, the gaincontrol in which the reception level converges to an ideal value fast(see FIG. 7) is carried out. If not, the gain control in which thereception level converges to an ideal value slow (see FIG. 9) is carriedout. It is noted that the process in FIG. 7 is approximately same as thesecond embodiment in FIG. 4 and the process in FIG. 9 is approximatelysame as the conventional example in FIG. 12. Therefore, description ofthese processes will not be repeated.

In this manner, in accordance with the fourth embodiment, the gainconvergence rate of AGC can be optimized by switching the gain controlmethod when it is determined that a signal is received from a new mobileterminal.

(Fifth Embodiment)

FIG. 10 is a functional block diagram functionally illustrating theradio apparatus in accordance with a fifth embodiment of the presentinvention.

A reception level detection unit 24 calculates a reception level withamplitude of RXIF signal before passing through quadrature detector 3.The remaining process is same as the gain control method in accordancewith the second embodiment. It is noted that a reception level can bedetected using RXIF signal also in the first, third and fourthembodiments, and a similar effect can be obtained as using RXIQ signal.

As described above, in accordance with the present invention, areception level can be converged to an ideal value by appropriatelycontrolling the convergence rate of AGC even under environments in whichthe reception level is largely fluctuated due to the effects of fading,reception of a signal from a new mobile terminal, and the like.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A radio apparatus carrying out automatic gain control for a signalfrom a mobile terminal unit, comprising: a gain control feedback circuitperforming automatic gain control on said signal depending on a level ofsaid signal; a fading rate estimation unit estimating a fading rate ofthe mobile terminal unit; and a step constant determiner determining anoptimum step constant which decides a gain of said gain control feedbackcircuit depending on the fading rate estimated by said fading rateestimation unit, wherein said gain control feedback circuit includes afeedback data calculator calculating feedback data which defines a gainof said gain control feedback circuit based on said signal level and theoptimum step constant as determined by said step constant determiner. 2.The radio apparatus according to claim 1, wherein said fading rateestimation unit calculates a reception response vector of the signalfrom the mobile terminal unit and estimates a fading rate from acorrelation value of said reception response vectors between frames. 3.The radio apparatus according to claim 1, wherein said step constantdeterminer decides an optimum step constant at the fading rate estimatedby said fading rate estimation unit based on an association table ofpreliminarily obtained fading rates and step constants.
 4. The radioapparatus according to claim 1, wherein said feedback data calculatorcalculates feedback data in a next frame by obtaining a value ofdifference between a value of said signal level and an ideal value,dividing the value of difference by a prescribed constant a number oftimes corresponding to said step constant, followed by multiplication bya prescribed coefficient, thereby calculating an amount of change infeedback data, and adding the amount of change to feedback data at thetime of receiving a previous frame.
 5. A radio apparatus carrying outautomatic gain control for a signal from a mobile terminal unit,comprising: a gain control feedback circuit performing automatic gaincontrol on said signal depending on a level of said signal; a receivedsignal strength detection unit detecting a received signal strength ofthe signal from the mobile terminal unit, at an antenna end of saidradio apparatus; a feedback data conversion unit carrying out conversionfrom the received signal strength detected by said received signalstrength detection unit to optimum feedback data which decides a gain ofsaid gain control feedback circuit based on an association table ofpreliminary obtained received signal strength and feedback data; and afeedback data switch unit selecting the optimum feedback data convertedby said feedback data conversion unit as an initial value of feedbackdata of said gain control feedback circuit.
 6. A radio apparatuscarrying out automatic gain control for a signal from a mobile terminalunit, comprising: a gain control feedback circuit performing automaticgain control on said signal depending on a level of said signal, saidgain control feedback circuit including a first gain control circuitsupplying first feedback data allowing the level of said signal toconverge to an ideal value slowly, and a second gain control circuitsupplying second feedback data allowing the level of said signal toconverge to an ideal value fast; a received signal strength detectionunit detecting a received signal strength of the signal from the mobileterminal unit at an antenna end of said radio apparatus; and a feedbackdata switch unit selecting said first feedback data or said secondfeedback data depending on a detection result from said received signalstrength detection unit.
 7. The radio apparatus according to claim 6,wherein said feedback data switch unit makes a switch from said firstfeedback data to said second feedback data when fluctuation of thereceived signal strength detected by said received signal strengthdetection unit decreases to a prescribed threshold or below or increasesto a prescribed threshold or above.
 8. A radio apparatus carrying outautomatic gain control for a signal from a mobile terminal unit,comprising: a gain control feedback circuit performing automatic gaincontrol on said signal depending on a level of said signal, said gaincontrol feedback circuit including a first gain control circuitsupplying first feedback data allowing the level of said signal toconverge to an ideal value slowly, and a second gain control circuitsupplying second feedback data allowing the level of said signal toconverge to an ideal value fast; a new terminal detector detectingwhether a signal is received from a new mobile terminal unit; and afeedback data switch unit selecting said first feedback data or saidsecond feedback data depending on a detection result from said newterminal detector.
 9. The radio apparatus according to claim 8, whereinsaid new terminal detector includes a demodulation circuit demodulatinga specific signal from a new mobile terminal unit, and an errordeterminer determining whether an error exists in the signal demodulatedby said demodulation circuit and if no error exists, determining that asignal is received from a new mobile terminal unit.
 10. The radioapparatus according to claim 9, wherein said error determiner determinesthat a signal is received from a new mobile terminal unit if a uniqueword error does not exist in the signal demodulated in said demodulationcircuit.
 11. The radio apparatus according to claim 9, wherein saiderror determiner determines that a signal is received from a new mobileterminal unit if the number of bit errors in a preamble portion is equalto or smaller than a prescribed threshold in the signal demodulated insaid demodulation circuit.
 12. The radio apparatus according to any ofclaims 1, 5, 6, or 8, wherein said gain control feedback circuitincludes a reception level detection unit detecting the level of saidsignal, and said reception level detection unit calculates an amplitudevalue for each IQ signal during a prescribed period of a received signaland regards a maximum amplitude value among said amplitude values as areception level of said received signal.
 13. The radio apparatusaccording to any of claims 1, 5, 6, or 8, wherein said gain controlfeedback circuit includes a reception level detection unit detecting thelevel of said signal, and said reception level detection unit calculatesan amplitude value for each IF signal during a prescribed period of areceived signal and regards a maximum amplitude value among saidamplitude values as a reception level of said received signal.
 14. Again control method in a radio apparatus carrying out automatic gaincontrol for a signal from a mobile terminal unit, comprising the stepsof: performing automatic gain control on said signal depending on alevel of said signal; estimating a fading rate of the mobile terminalunit; and determining an optimum step constant which decides a gain ofsaid automatic gain control, depending on said estimated fading rate,wherein said step of carrying out automatic gain control includes thestep of calculating feedback data which defines a gain of said automaticgain control, depending on said signal level and said optimum stepconstant as determined.
 15. The gain control method according to claim14, wherein said step of estimating a fading rate includes the step ofcalculating a reception response vector of the signal from the mobileterminal unit and estimating a fading rate from a correlation value ofsaid reception response vectors between frames.
 16. The gain controlmethod according to claim 14, wherein said step of determining anoptimum step constant includes the step of deciding an optimum stepconstant at said estimated fading rate based on an association table ofpreliminary obtained fading rates and step constants.
 17. The gaincontrol method according to claim 14, wherein said step of calculatingfeedback data includes the step of calculating feedback data in a nextframe by obtaining a value of difference between a value of said signallevel and an ideal value, dividing the value of difference by aprescribed constant a number of times corresponding to said stepconstant, followed by multiplication by a prescribed coefficient,thereby calculating an amount of change in feedback data, and adding theamount of change to feedback data at the time of receiving a previousframe.
 18. A gain control method in a radio apparatus carrying outautomatic gain control for a signal from a mobile terminal unit,comprising the steps of: performing automatic gain control on saidsignal depending on a level of said signal; detecting a received signalstrength of the signal from the mobile terminal unit at an antenna endof said radio apparatus; carrying out conversion from said detected,received signal strength to optimum feedback data which decides a gainof said automatic gain control based on an association table ofpreliminarily obtained received signal strengths and feedback data; andselecting said converted optimum feedback data as an initial value offeedback data.
 19. A gain control method in a radio apparatus carryingout automatic gain control for a signal from a mobile terminal unit,comprising the steps of: performing automatic gain control on saidsignal depending on a level of said signal, said step of performingautomatic gain control including a first step of supplying firstfeedback data allowing the level of said signal to converge to an idealvalue slowly, and a second step of supplying second feedback dataallowing the level of said signal to converge to an ideal value fast;detecting a received signal strength of the signal from the mobileterminal unit, at an antenna end of said radio apparatus; and selectingsaid first feedback data or said second feedback data depending on aresult of said detection.
 20. The gain control method according to claim19, wherein said step of selecting feedback data includes the step ofmaking a switch from said first feedback data to said second feedbackdata when fluctuation of said detected, received signal strengthdecreases to a prescribed threshold or below or increases to aprescribed threshold or above.
 21. A gain control method in a radioapparatus carrying out automatic gain control for a signal from a mobileterminal unit, comprising the steps of: performing automatic gaincontrol on said signal depending on a level of said signal, said step ofperforming automatic gain control including a first step of supplyingfirst feedback data allowing the level of said signal to converge to anideal value slowly, and a second step of supplying second feedback dataallowing the level of said signal to converge to an ideal value fast;detecting whether a signal is received from a new mobile terminal unit;and selecting said first feedback data or said second feedback datadepending on a result of said detection.
 22. The gain control methodaccording to claim 21, wherein said step of detecting a new mobileterminal unit includes the steps of demodulating a specific signal froma new mobile terminal unit, and determining whether an error exists inthe demodulated signal and if no error exists, determining that a signalis received from a new mobile terminal unit.
 23. The gain control methodaccording to claim 22, wherein said step of determining whether an errorexists includes the step of determining that a signal has been receivedfrom a new mobile terminal unit when a unique word error does not existsin said demodulated signal.
 24. The gain control method according toclaim 22, wherein said step of determining whether an error existsincludes the step of determining that a signal has been received from anew mobile terminal unit if the number of bit errors in a preambleportion is equal to or smaller than a prescribed threshold in saiddemodulated signal.
 25. The gain control method according to any ofclaims 14, 18, 19, or 21, wherein said step of performing automatic gaincontrol includes the step of detecting the level of said signal, andsaid step of detecting the level of said signal has the step ofcalculating an amplitude value for each IQ signal during a prescribedperiod of a received signal and regarding a maximum amplitude valueamong said amplitude values as a reception level of said receivedsignal.
 26. The gain control method according to any of claims 14, 18,19, or 21, wherein said step of performing automatic gain controlincludes the step of detecting the level of said signal, and said stepof detecting the level of said signal has the step of calculating anamplitude value for each IF signal during a prescribed period of areceived signal and regarding a maximum amplitude value among saidamplitude values as a reception level of said received signal.
 27. Aradio apparatus carrying out automatic gain control for a signal from amobile terminal unit, comprising: a gain control feedback circuitperforming automatic gain control on said signal depending on a level ofsaid signal; a received signal strength detection unit detecting areceived signal strength of the signal from the mobile terminal unit, atan antenna end of said radio apparatus; a feedback data conversion unitcarrying out conversion from the received signal strength detected bysaid received signal strength detection unit to optimum feedback datawhich decides a gain of said gain control feedback circuit based onpreliminary obtained received signal strength and feedback data; and afeedback data switch unit selecting the optimum feedback data convertedby said feedback data conversion unit as an initial value of feedbackdata of said gain control feedback circuit.
 28. A radio apparatuscarrying out automatic gain control for a signal from a terminal unit,comprising: a gain control feedback circuit performing automatic gaincontrol on said signal depending on a level of said signal; a receivedsignal strength detection unit detecting a received signal strength ofthe signal from the terminal unit, at an antenna end of said radioapparatus; a feedback data conversion unit carrying out conversion fromthe received signal strength detected by said received signal strengthdetection unit to optimum feedback data which decides a gain of saidgain control feedback circuit based on an association table ofpreliminary obtained received signal strength and feedback data; and afeedback data switch unit selecting the optimum feedback data convertedby said feedback data conversion unit as an initial value of feedbackdata of said gain control feedback circuit.
 29. A radio apparatuscarrying out automatic gain control for a signal from a mobile terminalunit, comprising: a gain control feedback circuit performing automaticgain control on said signal depending on a level of said signal; areceived signal strength detection unit detecting a received signalstrength of the signal from the mobile terminal unit, at said radioapparatus; a feedback data conversion unit carrying out conversion fromthe received signal strength detected by said received signal strengthdetection unit to optimum feedback data which decides a gain of saidgain control feedback circuit based on an association table ofpreliminary obtained received signal strength and feedback data; and afeedback data switch unit selecting the optimum feedback data convertedby said feedback data conversion unit as an initial value of feedbackdata of said gain control feedback circuit.
 30. A radio apparatuscarrying out automatic gain control for a signal from a terminal unit,comprising: a gain control feedback circuit performing automatic gaincontrol on said signal depending on a level of said signal; a receivedsignal strength detection unit detecting a received signal strength ofthe signal from the terminal unit, at an antenna end of said radioapparatus; a feedback data conversion unit carrying out conversion fromthe received signal strength detected by said received signal strengthdetection unit to optimum feedback data which decides a gain of saidgain control feedback circuit based on preliminary obtained receivedsignal strength and feedback data; and a feedback data switch unitselecting the optimum feedback data converted by said feedback dataconversion unit as an initial value of feedback data of said gaincontrol feedback circuit.
 31. A radio apparatus carrying out automaticgain control for a signal from a mobile terminal unit, comprising: again control feedback circuit performing automatic gain control on saidsignal depending on a level of said signal; a received signal strengthdetection unit detecting a received signal strength of the signal fromthe mobile terminal unit, at said radio apparatus; a feedback dataconversion unit carrying out conversion from the received signalstrength detected by said received signal strength detection unit tooptimum feedback data which decides a gain of said gain control feedbackcircuit based on preliminary obtained received signal strength andfeedback data; and a feedback data switch unit selecting the optimumfeedback data converted by said feedback data conversion unit as aninitial value of feedback data of said gain control feedback circuit.32. A radio apparatus carrying out automatic gain control for a signalfrom a terminal unit, comprising: a gain control feedback circuitperforming automatic gain control on said signal depending on a level ofsaid signal; a received signal strength detection unit detecting areceived signal strength of the signal from the terminal unit, at saidradio apparatus; a feedback data conversion unit carrying out conversionfrom the received signal strength detected by said received signalstrength detection unit to optimum feedback data which decides a gain ofsaid gain control feedback circuit based on an association table ofpreliminary obtained received signal strength and feedback data; and afeedback data switch unit selecting the optimum feedback data convertedby said feedback data conversion unit as an initial value of feedbackdata of said gain control feedback circuit.
 33. A radio apparatuscarrying out automatic gain control for a signal from a terminal unit,comprising: a gain control feedback circuit performing automatic gaincontrol on said signal depending on a level of said signal; a receivedsignal strength detection unit detecting a received signal strength ofthe signal from the terminal unit, at said radio apparatus; a feedbackdata conversion unit carrying out conversion from the received signalstrength detected by said received signal strength detection unit tooptimum feedback data which decides a gain of said gain control feedbackcircuit based on preliminary obtained received signal strength andfeedback data; and a feedback data switch unit selecting the optimumfeedback data converted by said feedback data conversion unit as aninitial value of feedback data of said gain control feedback circuit.34. A gain control method in a radio apparatus carrying out automaticgain control for a signal from a mobile terminal unit, comprising thesteps of: performing automatic gain control on said signal depending ona level of said signal; detecting a received signal strength of thesignal from the mobile terminal unit at an antenna end of said radioapparatus; carrying out conversion from said detected, received signalstrength to optimum feedback data which decides a gain of said automaticgain control based on preliminarily obtained received signal strengthsand feedback data; and selecting said converted optimum feedback data asan initial value of feedback data.
 35. A gain control method in a radioapparatus carrying out automatic gain control for a signal from aterminal unit, comprising the steps of: performing automatic gaincontrol on said signal depending on a level of said signal; detecting areceived signal strength of the signal from the terminal unit at anantenna end of said radio apparatus; carrying out conversion from saiddetected, received signal strength to optimum feedback data whichdecides a gain of said automatic gain control based on an associationtable of preliminarily obtained received signal strengths and feedbackdata; and selecting said converted optimum feedback data as an initialvalue of feedback data.
 36. A gain control method in a radio apparatuscarrying out automatic gain control for a signal from a mobile terminalunit, comprising the steps of: performing automatic gain control on saidsignal depending on a level of said signal; detecting a received signalstrength of the signal from the mobile terminal unit at said radioapparatus; carrying out conversion from said detected, received signalstrength to optimum feedback data which decides a gain of said automaticgain control based on an association table of preliminarily obtainedreceived signal strengths and feedback data; and selecting saidconverted optimum feedback data as an initial value of feedback data.37. A gain control method in a radio apparatus carrying out automaticgain control for a signal from a terminal unit, comprising the steps of:performing automatic gain control on said signal depending on a level ofsaid signal; detecting a received signal strength of the signal from theterminal unit at an antenna end of said radio apparatus; carrying outconversion from said detected, received signal strength to optimumfeedback data which decides a gain of said automatic gain control basedon preliminarily obtained received signal strengths and feedback data;and selecting said converted optimum feedback data as an initial valueof feedback data.
 38. A gain control method in a radio apparatuscarrying out automatic gain control for a signal from a mobile terminalunit, comprising the steps of: performing automatic gain control on saidsignal depending on a level of said signal; detecting a received signalstrength of the signal from the mobile terminal unit at said radioapparatus; carrying out conversion from said detected, received signalstrength to optimum feedback data which decides a gain of said automaticgain control based on preliminarily obtained received signal strengthsand feedback data; and selecting said converted optimum feedback data asan initial value of feedback data.
 39. A gain control method in a radioapparatus carrying out automatic gain control for a signal from aterminal unit, comprising the steps of: performing automatic gaincontrol on said signal depending on a level of said signal; detecting areceived signal strength of the signal from the terminal unit at saidradio apparatus; carrying out conversion from said detected, receivedsignal strength to optimum feedback data which decides a gain of saidautomatic gain control based on an association table of preliminarilyobtained received signal strengths and feedback data; and selecting saidconverted optimum feedback data as an initial value of feedback data.40. A gain control method in a radio apparatus carrying out automaticgain control for a signal from a terminal unit, comprising the steps of:performing automatic gain control on said signal depending on a level ofsaid signal; detecting a received signal strength of the signal from theterminal unit at said radio apparatus; carrying out conversion from saiddetected, received signal strength to optimum feedback data whichdecides a gain of said automatic gain control based on preliminarilyobtained received signal strengths and feedback data; and selecting saidconverted optimum feedback data as an initial value of feedback data.41. A radio apparatus carrying out automatic gain control for a signalfrom a terminal unit, comprising: a gain control feedback circuitperforming automatic gain control on said signal depending on a level ofsaid signal; a fading rate estimation unit estimating a fading rate ofthe terminal unit; and a step constant determiner determining an optimumstep constant which decides a gain of said gain control feedback circuitdepending on the fading rate estimated by said fading rate estimationunit, wherein said gain control feedback circuit includes a feedbackdata calculator calculating feedback data which defines a gain of saidgain control feedback circuit based on said signal level and the optimumstep constant as determined by said step constant determiner.
 42. Theradio apparatus according to claim 41, wherein said fading rateestimation unit calculates a reception response vector of the signalfrom the terminal unit and estimates a fading rate from a correlationvalue of said reception response vectors between frames.
 43. A radioapparatus carrying out automatic gain control for a signal from aterminal unit, comprising: a gain control feedback circuit performingautomatic gain control on said signal depending on a level of saidsignal, said gain control feedback including a first gain controlcircuit supplying first feedback data allowing the level of said signalto converge to an ideal value slowly, and a second gain control circuitsupplying second feedback data allowing the level of said signal toconverge to an ideal value fast; a received signal strength detectionunit detecting a received signal strength of the signal from theterminal unit at an antenna end of said radio apparatus; and a feedbackdata switch unit selecting said first feedback data or said secondfeedback data depending on a detection result from said received signalstrength detection unit.
 44. A radio apparatus carrying out automaticgain control for a signal from a terminal unit, comprising: a gaincontrol feedback circuit performing automatic gain control on saidsignal depending on a level of said signal, said gain control feedbackcircuit including a first gain control circuit supplying first feedbackdata allowing the level of said signal to converge to an ideal valueslowly, and a second gain control circuit supplying second feedback dataallowing the level of said signal to converge to an ideal value fast; anew terminal detector detecting whether a signal is received from a newterminal unit; and a feedback data switch unit selecting said firstfeedback data or said second feedback data depending on a detectionresult from said new terminal detector.
 45. The radio apparatusaccording to claim 44, wherein said new terminal detector includes ademodulation circuit demodulating a specific signal from a new terminalunit, and an error determiner determining whether an error exists in thesignal demodulated by said demodulation circuit and if no error exists,determining that a signal is received from a new terminal unit.
 46. Theradio apparatus according to claim 45, wherein said error determinerdetermines that a signal is received from a new terminal unit if aunique word error does not exist in the signal demodulated in saiddemodulation circuit.
 47. The radio apparatus according to claim 45wherein said error determiner determines that a signal is received froma new terminal unit if the number of bit errors in a preamble portion isequal to or smaller than a prescribed threshold in the signaldemodulated in said demodulation circuit.
 48. A gain control method in aradio apparatus carrying out automatic gain control for a signal from aterminal unit, comprising the steps of: performing automatic gaincontrol on said signal depending on a level of said signal; estimating afading rate of the terminal unit; and determining an optimum stepconstant which decides a gain of said automatic gain control, dependingon said estimated fading rate, wherein said step of carrying outautomatic gain control includes the step of calculating feedback datawhich defines a gain of said automatic gain control, depending on saidsignal level and said optimum step constant as determined.
 49. The gaincontrol method according to claim 48 wherein said step of estimating afading rate includes the step of calculating a reception response vectorof the signal from the terminal unit and estimating a fading rate from acorrelation value of said reception response vectors between frames. 50.A gain control method in a radio apparatus carrying out automatic gaincontrol for a signal from a terminal unit, comprising the steps of:performing automatic gain control on said signal depending on a level ofsaid signal, said step of performing automatic gain control including afirst step of supplying first feedback data allowing the level of saidsignal to converge to an ideal value slowly, and a second step ofsupplying second feedback data allowing the level of said signal toconverge to an ideal value fast; detecting a received signal strength ofthe signal from the terminal unit, at an antenna end of said radioapparatus; and selecting said first feedback data or said secondfeedback data depending on a result of said detection.
 51. A gaincontrol method in a radio apparatus carrying out automatic gain controlfor a signal from a terminal unit, comprising the steps of: performingautomatic gain control on said signal depending on a level of saidsignal, said step of performing automatic gain control including a firststep of supplying first feedback data allowing the level of said signalto converge to an ideal value slowly, and a second step of supplyingsecond feedback data allowing the level of said signal to converge to anideal value fast; detecting whether a signal is received from a newterminal unit; and selecting said first feedback data or said secondfeedback data depending on a result of said detection.
 52. The gaincontrol method according to claim 51 wherein said step of detecting anew terminal unit includes the steps of demodulating a specific signalfrom a new terminal unit, and determining whether an error exists in thedemodulated signal and if no error exists, determining that a signal isreceived from a new terminal unit.
 53. The gain control method accordingto claim 51, wherein said step of determining whether an error existsincludes the step of determining that a signal has been received from anew terminal unit when a unique word error does not exist in saiddemodulated signal.
 54. The gain control method according to claim 51,wherein said step of determining whether an error exists includes thestep of determining that a signal has been received from a new terminalunit if the number of bit errors in a preamble portion is equal to orsmaller than a prescribed threshold in said demodulated signal.